1. Field of the Invention
The preset invention relates to a wavelength conversion laser light source for converting a laser beam emitted from a laser light source using the non-linear optical effects, and also to a laser light source device and a two-dimensional image display device adopting the same.
2. Description of the Background Art
Conventionally, a variety of wavelength conversion laser light sources have been developed and made into practical applications, wherein a visible laser beam is obtained, such as a green light or an ultraviolet ray which is obtained by further converting the green light, etc., through the wavelength conversion using the non-linear optical effects of a laser beam emitted from the Nd:YAG laser, or the Nd:YVO4 laser. These converted light beams are used for laser processing, or a laser display, etc.
FIG. 2 shows a typical structure of a conventional wavelength conversion laser light source using the non-linear optical effect. In order to obtain the non-linear optical effect, it is required to adopt the non-linear optical crystals having the birefringence. Examples of such non-linear optical crystals having the birefringence include: LiB3O5 (LBO: lithium triborate), KTiOPO4 (KTP: Potassium Titanyl Phosphate), CsLiB6O10 (CLBO: Cesium Lithium Borate); or LiNbO3 (PPLN: Lithium Tantalate), and LiTaO3 (PPLT: Lithium Tantalate) having a periodical polarization inversion structure, etc.
As shown in FIG. 2, a wavelength conversion laser light source 200 includes a fundamental wave light source 101, a collective lens 108, a non-linear optical crystals (wavelength conversion element) 109, a re-collimating lens 111, a wavelength-dividing mirror, a temperature holder 201 such as a heater or the like for holding the temperature of the non-linear optical crystals constant, a power supply 114, a control unit 225 for controlling a laser output, and a temperature controller (not shown) provided in the control unit 225 for controlling the temperature of the non-linear optical crystals. For the fundamental wave light source 101, Nd:YAG laser, Nd:YVO4 laser, fiber laser using Yb doped fiber having a wavelength of 1.06 μm are generally used.
Here, the actual operations will be explained, which generate the second harmonic wave having a wavelength of 0.532 μm, which is around ½ of wavelength (1.06 μm) of the fundamental wave.
The laser beam having a wavelength of 1.06 μm as emitted from the fundamental wave light source 101 is converged into the non-linear optical crystals 109 by the collective lens 108. Here, the non-linear optical crystals 109 needs to have the refractive index for the light having the wavelength of 1.06 μm matched with the refractive index for the light having the wavelength of 0.532 μm to be generated (phase matching condition). Generally, the refractive index for the crystals varies according to temperature conditions of the crystals. Therefore, the temperature of the crystals needs to be maintained constant. For this reasons, the non-linear optical crystals are placed in the temperature holder 201, and are maintained at a predetermined temperature suited for the kind of the crystals. For example, when adopting the LBO crystals, in order to obtain the type-1 non-critical phase matching (the phase matching state), the LBO crystals need to be maintained at a temperature in a range of from 148° C. to 150° C. as disclosed in Japanese Unexamined Patent Publication No. 2007-516600.
On the other hand, when adopting LiNbO3 crystals having a periodical polarization inversion structure, it is possible to set the temperature and the wavelength for the phase matching condition by selecting the period for the periodical polarization inversion structure. In this case, however, it is necessary to keep the temperature and the fundamental harmonic wavelength of non-linear optical crystal 109 constant in order to maintain the phase matching condition as disclosed in Japanese Unexamined Patent Publication No. 2007-73552.
However, it has been found that for some kinds of the non-linear optical crystals to be adopted as the wavelength conversion element, the temperature of the element is raised by absorbing the fundamental wave and the harmonic wave as generated, which makes the phase matching temperature (wavelength) vary according to the output level of the harmonic wave, thereby presenting a problem in that a high conversion efficiency cannot be realized.
For the temperature holder 201, a Peltier device or a heater is used conventionally. However, when adopting such temperature holder 201, it takes minute order time for the non-linear optical crystal 109 to arrive to the target temperature after starting the wavelength conversion laser source. Therefore, there is a problem that the wavelength conversion laser source that has temperature holder 201 with a Peltier device or a heater is unsuitable for the usage of the optical source for the display from which lighting is demanded momentarily etc.
When the temperature control is performed by using the Peltier device as the temperature holder 201 at the temperature in vicinity of the room temperature (20° C.-30° C.), it is possible to shorten the start-up time to some degree, and to shorten the time of arrival to the target temperature. However, when the room temperature is higher than the holding temperature of non-linear optical crystal 109 and humidity is high, a problem arises in that dew condensation is generated on the surface of the non-linear optical crystal.
Even when a rapid start-up is to be performed by adopting the temperature holder 201 with a mass heater, some time is still required to stabilize the temperature of non-linear optical crystal 109. Moreover, there is a problem in respect of the reliability as the non-linear optical crystal 109 may be damaged due to thermal stress.